After 3 years of preliminary studies from age 10 onwards, in 1561 Suárez matriculated at the University of Salamanca, and studied Law. In 1564, at age sixteen, Suárez entered the Society of Jesus in Salamanca. In August 1566, Suárez took his first vows as a Jesuit; he then began in October 1566 to study Theology at Salamanca. It seems he was not a promising student at first; in fact, he nearly gave up his matters of study after failing the entrance exam twice. After passing the exam at third attempt, though, things changed.

In 1570, with the completion of his course, Suárez began to teach Philosophy, first at Salamanca as a Scholastic tutor, and then as a professor in the Jesuit college at Segovia. He was ordained in March 1572 in Segovia. He continued to teach Philosophy in Segovia until, in September 1574, he moved to the Jesuit College in Valladolid to teach Theology, a subject he would then teach for the rest of his life. He taught in a succession of different places: Avila (1575), Segovia (1575), Valladolid (1576) Rome (1580–85), Alcalá (1585–92) and Salamanca (1592–97). In 1597, he moved to Coimbra, some years after the accession of the Spanish (elder line) House of Habsburg to the Portuguese Throne, to take up the principal chair of Theology at the University of Coimbra. He remained there, aside from a brief time teaching at Rome, until his death in 1617.

He wrote on a wide variety of subjects, producing a vast amount of work (his complete works in Latin amount to twenty-six volumes). Suárez writings include treatises on law, the relationship between Church and State, metaphysics, and theology. He is considered the godfather of International Law. His Disputationes metaphysicae (Metaphysical Disputations) were widely read in Europe during the 17th century and are considered by some scholars to be his most profound work.

In 1679 Pope Innocent XI publicly condemned sixty-five casuist propositions, taken chiefly from the writings of Escobar, Suárez and others, mostly Jesuit, theologians as propositiones laxorum moralistarum and forbade anyone to teach them under penalty of excommunication.[4]

His most important philosophical achievements were in metaphysics and the philosophy of law. Suárez may be considered the last eminent representative of the scholasticism. He adhered to a moderate form of Thomism and developed metaphysics as a systematic enquiry.

For Suárez, metaphysics was the science of real essences (and existence); it was mostly concerned with real being rather than conceptual being, and with immaterial rather than with material being. He held (along with earlier scholastics) that essence and existence are the same in the case of God (see ontological argument), but disagreed with Aquinas and others that the essence and existence of finite beings are really distinct. He argued that in fact they are merely conceptually distinct: rather than being really separable, they can only logically be conceived as separate.

On the vexed subject of universals, he endeavored to steer a middle course between the realism of Duns Scotus and the nominalism of William of Occam. His position is a little bit closer to nominalism than that of Thomas Aquinas.[5] Sometimes he is classified as a moderate nominalist, but his admitting of objective precision (praecisio obiectiva) ranks him with moderate realists. The only veritable and real unity in the world of existences is the individual; to assert that the universal exists separately ex parte rei would be to reduce individuals to mere accidents of one indivisible form. Suárez maintains that, though the humanity of Socrates does not differ from that of Plato, yet they do not constitute realiter one and the same humanity; there are as many "formal unities" (in this case, humanities) as there are individuals, and these individuals do not constitute a factual, but only an essential or ideal unity ("In such a way, that many individuals, which are said to be of the same nature, are so: only through the operation of the intellect, not through a substance or essence of things which unites them").[6] The formal unity, however, is not an arbitrary creation of the mind, but exists "in the nature of the thing, prior [ontologically] to any operation of the intellect".[7]

His metaphysical work, giving a remarkable effort of systematisation, is a real history of medieval thought, combining the three schools available at that time: Thomism, Scotism and Nominalism. He is also a deep commentator of Arabic or high medieval works. He enjoyed the reputation of being the greatest metaphysician of his time. He thus founded a school of his own,Suarism or Suarezianism, the chief characteristic principles of which are:

a distinctio rationis ratiocinatae between the essence and the existence of created beings

the possibility of spiritual substance only numerically distinct from one another

ambition for the hypostatic union as the sin of the fallen angels

the Incarnation of the Word, even if Adam had not sinned

the solemnity of the vow only in ecclesiastical law

the system of Congruism that modifies Molinism by the introduction of subjective circumstances, as well as of place and of time, propitious to the action of efficacious grace, and with predestination ante praevisa merita

the possibility of holding one and the same truth by both science and faith

the belief in Divine authority contained in an act of faith

the production of the body and blood of Christ by transubstantiation as constituting the Eucharistic sacrifice

Suárez made an important investigation of being, its properties and division in Disputationes Metaphysicae (1597), which influenced the further development of theology within Catholicism. In the second part of the book, disputations 28-53, Suárez fixes the distinction between ens infinitum (God) and ens finitum (created beings). The first division of being is that between ens infinitum and ens finitum. Instead of dividing being into infinite and finite, it can also be divided into ens a se and ens ab alio, i.e., being that is from itself and being that is from another. A second distinction corresponding to this one:ens necessarium and ens contingens, i.e., necessary being and contingent being. Still another formulation of the distinction is between ens per essentiam and ens per participationem, i.e., being that exists by reason of its essence and being that exists only by participation in a being that exists on its own (eigentlich). A further distinction is between ens increatum and ens creatum, i.e., uncreated being and created, or creaturely, being. A final distinction is between being as actus purus and being as ens potentiale, i.e., being as pure actuality and being as potential being. Suárez decided in favor of the first classification of the being into ens infinitum and ens finitum as the most fundamental, in connection with which he accords the other classifications their due. In the last disputation 54 Suárez deals with entia rationis (beings of reason), which are impossible intentional objects, i.e. objects that are created by our minds but cannot exist in actual reality.[9]

In theology, Suárez attached himself to the doctrine of Luis Molina, the celebrated Jesuit professor of Évora. Molina tried to reconcile the doctrine of predestination with the freedom of the human will and the predestinarian teachings of the Dominicans by saying that the predestination is consequent upon God's foreknowledge of the free determination of man's will, which is therefore in no way affected by the fact of such predestination. Suárez endeavoured to reconcile this view with the more orthodox doctrines of the efficacy of grace and special election, maintaining that, though all share in an absolutely sufficient grace, there is granted to the elect a grace which is so adapted to their peculiar dispositions and circumstances that they infallibly, though at the same time quite freely, yield themselves to its influence. This mediatizing system was known by the name of "congruism."

Here, Suárez's main importance stems probably from his work on natural law, and from his arguments concerning positive law and the status of a monarch. In his extensive work, Tractatus de legibus ac deo legislatore (reprinted, London, 1679), he is to some extent the precursor of Grotius and Pufendorf, in making an important distinction between natural law and international law, which he saw as based on custom. Though his method is throughout scholastic, he covers the same ground, and Grotius speaks of him with great respect. The fundamental position of the work is that all legislative as well as all paternal power is derived from God, and that the authority of every law resolves itself into His. Suárez denies the patriarchal theory of government and the divine right of kings founded upon it, doctrines popular at that time in England and to some extent on the Continent. He argued against the sort of social contract theory that became dominant among early-modern political philosophers such as Thomas Hobbes and John Locke, but some of his thinking found echoes in the more liberal, Lockean contract theorists.[citation needed]

He argued that human beings have a social nature bestowed upon them by God, and this includes the potential to make laws. However, when a political society is formed, the authority of the state is not of divine but of human origin; therefore, its nature is chosen by the people involved, and their natural legislative power is given to the ruler.[10] Because they gave this power, they have the right to take it back and to revolt against a ruler, only if the ruler behaves badly towards them, and they must act moderately and justly. In particular, the people must refrain from killing the ruler, no matter how tyrannical he may have become. If a government is imposed on people, on the other hand, they have the right to defend themselves by revolting against it and even kill the tyrannical ruler.[citation needed]

In 1613, at the instigation of Pope Paul V, Suárez wrote a treatise dedicated to the Christian princes of Europe, entitled Defensio catholicae fidei contra anglicanae sectae errores ("Defense of the Universal Catholic Faith Against the Errors of the Anglican Sect").[11] This was directed against the oath of allegiance which James I required from his subjects.

James (himself a talented scholar) caused it to be burned by the common hangman and forbade its perusal under the 'severest penalties, complaining bitterly to Philip III of Spain for harbouring in his dominions a declared enemy of the throne and majesty of kings.

The contributions of Suarez to metaphysics and theology exerted significant influence over 17th and 18th century scholastic theology among both Roman Catholics and Protestants.[12]

Thanks in part to the strength of Suárez's Jesuit order, his Disputationes Metaphysicae was widely taught in the Catholic schools of Spain, Portugal and Italy.

It also spread from these schools to many Lutheran universities in Germany, where the text was studied especially by those who favoured Melanchthon rather than Luther's attitude towards philosophy. In a number of seventeenth-century Lutheran universities the Disputationes served as a textbook in philosophy.

In a similar way, Suárez had major influence in the Reformed tradition of German and Dutch schools for both metaphysics and law, including international law. His work was highly praised, for example, by Hugo Grotius (1583-1645).

The views of Suarez upon the human origin of political order, and his defense of tyrannicide emanating from popular dissent were heavily criticized by English philosopher Robert Filmer in his work Patriarcha, Or the Natural Power of Kings. Filmer believed the Calvinists and the Papists like Suarez to be dangerous opponents of divine right monarchy, legitimized by the supremacy of fathers upon their offspring, which Filmer claimed could be traced back to Adam.[17]

In the 18th century, the Venice edition of Opera Omnia in 23 volumes in folio (1740–1751) appeared, followed by the Parisian Vivès edition, 26 volumes + 2 volumes of indices (1856–1861); in 1965 the Vivés edition of the Disputationes Metaphysicae (volls. 25-26) was reprinted by Georg Olms, Hildesheim. From 1597 to 1636 the Disputationes Metaphysicae were published in seventeen editions; no modern edition of Suárez's complete works is yet available and only few of Suárez's Disputations have been translated into English.